The present invention generally relates to devices and methods for removing target contaminant compounds from a liquid, such as water. More particularly, the present invention relates to a method for enhancing microbial/enzyme robustness and performance by directly supplying them with electrons/electron acceptor rich environment, such as in an electrochemical bioreactor (EBR), to facilitate transformation and/or removal of target compounds from a liquid at a significantly higher rate and greater efficiency possible than in conventional bioreactors with significantly less biomass production.
Metals and other inorganics like arsenic, selenium, mercury, cadmium, chromium, nitrogen, etc. are difficult to remove to levels that meet current drinking water and discharge criteria in many countries. For example, in the United States, the 2006 maximum arsenic level in drinking waters was set at 10 ppb; this may soon be the case in other countries. Maximum contaminant levels (MCL) of metals in drinking water in the United States can range 0.0005 to 10 mg/L, and can be even lower. Commonly regulated metals and inorganics include antimony, arsenic, barium, cadmium, chromium, copper, cyanide, fluoride, lead, mercury, nitrate, nitrite, selenium, sulfate, thallium and zinc.
There are various kinds of treatment methods for metal, inorganics, and organics removal. Physical, chemical and biological technologies used to treat metal and inorganic-contaminated water including: membrane treatments such as reverse osmosis and nano filtration; ion exchange and sorption; physical/chemical precipitations and physical separations; and various biological treatments that usually refer to the use of bacteria in engineered reactor systems for effecting the removal and/or transformation of contaminants through the addition of nutrients.
All chemical reactions or transformations require the exchange of electrons; this occurs through well-documented oxidation-reduction (ORP) reactions. ORP reactions are termed half reactions that require a loss of electrons (oxidation reactions) and a second set of concurrent reactions that require a gain of electrons (reduction reactions). These reactions are measured through reduction potential, sometimes referred to as redox potential or oxidation/reduction potential, ORP as measured in volts or Eh(V), voltage or concentration of electrons and pH a measure of the concentration of hydrogen ions. This means that electrons must be added from some reactants and removed from other reactants for the reactions to take place. ORP reactions also occur at different energy yields or requirements; the more positive the ORP the greater the energy yield. The more negative the ORP, the greater the energy requirement. There is also, in most ORP reactions, an energy of activation that is needed to have the reaction move toward the reactant products. This energy can be supplied to the system in various manners, for example heat, chemical electron donors such as sugars, or easily available electrons.
Biological wastewater treatments are based on microbial transformations of contaminants (reactants). Microbes mediate the removal of metal and inorganic contaminants through electron transfer (redox processes). For example, denitrification and selenium reduction can be described by the following redox reactions:
                                          NO            3            -                    +                      5            ⁢                          e              -                                +                      6            ⁢                          H              +                                      ->                                            1              2                        ⁢                          N              2                                +                      3            ⁢                          H              2                        ⁢            O                                              (        1        )                                                      SeO            4                          2              -                                +                      6            ⁢                          e              -                                +                      8            ⁢                          H              +                                      ->                              Se                          (              s              )                                +                      4            ⁢                          H              2                        ⁢            O                                              (        2        )            
Both biotransformations shown in reactions 1 and 2 occur under anaerobic, reductive conditions, thus require low dissolved oxygen (DO) levels and a negative ORP (oxidation/reduction potential) environment. Eleven electrons are needed to reduce one molecule of selenate and one molecule of nitrate to elemental selenium and nitrogen gas. Other co-contaminants or reactants present, such as arsenate and oxygen, would add to the electron demand (all other electron acceptors present that accept electrons at the same or lower energy level).
As an example of a chemical microbial electron supply, glucose is often used as a cost-effective microbial electron donor and ORP adjustment chemical. In microbial biotreatment systems one molecule of glucose can provide up to 24 electrons with complete metabolism under optimal conditions (usually measured in 24 to 72 hours). In environmental and other applications, this efficiency or the amount of available electrons actually realized is usually considerably less because energy is required for metabolism and is lost due to system influences or slowed due to temperature. Furthermore, only a few of these electrons are available within the first 6 to 8 hours, requiring a large excess of organic electron donors to approach the desired number of electrons needed for microbial mediated contaminant removals in the 6 to 8 hour time frame needed to keep bioreactor sizes reasonably small.
In conventional biological treatment systems, excess nutrients are added to the system to provide electrons needed for 1) microbial growth, 2) various contaminant biotransformations (reactants such as metals and inorganics), 3) ORP adjustment; 4) to compensate for overall system sensitivity, and 5) to supplement decreased metabolic and enzymatic rates, such as at low temperatures that slow metabolism and reaction rates resulting in electrons being provided at slower rates or in electron ‘needy’ environments that yield fewer electrons for microbial use. As an example, nutrients added to a biotreatment system only yield electrons for microbial use upon metabolism. This requires addition of excess nutrients that adds significantly to both capital expenditures (CAPEX) and operating expenditures (OPEX) costs. The use of excess nutrients results in higher microbial nutrient consumption directed to the production of greater numbers of microbial cells (excessive biomass) due to the provision of carbon, nitrogen, phosphate, and/or sulfur with organic nutrients; excess biomass must be removed and properly disposed of, as it will contain contaminants found in the system, this also increases CAPEX and OPEX costs.
Accordingly, there is a continuing need for methods and systems for effectively and efficiently removing targeted contaminant compounds from liquids, such as drinking water and other water sources. The present invention fulfills these needs, and provides other related advantages.